Please clarify, where do you see this, are you on mobile, the web version, are you adding a task or?
I see this on web version, if I click on the three points at the end of a task, means in the context menu after "Erinnerungen" in the german version.
Can you send a screenshot of it to firstname.lastname@example.org
I'm also wondering what this means...? Is it just to mark a task as done or does this have an additional function?
Technically, this option completes the task just as if you'd click on the check box, but what makes it different from the check box is that it directly forces the task to move to history. This is useful if you have a recurring task and don't want it to recur anymore or a sub-task which you want to remove from the list right-away.
The alternative to this option is the SHIFT key. When you hold SHIFT while checking off a task through the check box, it will also force the task to move to history even if it's a sub-task or a recurring task.